In many deltaic areas of the world the ocean bottom is formed very rapidly by river silt deposits which accumulate, and which can exceed a build-up of as much as a foot per year. As a result of the rapid deposition rate and the fine particles being deposited, sections of the deposits are underconsolidated and remain in a flocculated condition. Further, such build-ups are characterized by very high water content and very low strength.
When deposits of this nature are acted upon by pressure differentials such as those caused by large waves, the soft materials often slide down the Continental Shelf, thereby forming a slide channel. Such a slide is similar in many respects to a river channel. The channel can be several hundred, or even thousands of feet wide, more than 100 feet deep, and extend for its length over several miles. Further, these slides tend to occur in water depths up to 200 to 300 feet due to the normal reduction of wave pressure with water depth.
Conventional offshore fixedly positioned drilling and production platforms, and similar fixed marine structures will not resist the loss of the horizontal soil support coupled with the very high lateral loads which result from the above noted sliding mass of soil. From experience, it is known that several marine platforms and ocean floor pipelines have been destroyed. These were experienced in the Gulf of Mexico as well as in other areas of the world.
The invention in brief therefore contemplates a marine structure or platform that is capable of being floated to an offshore working site, and thereafter being controllably submerged to the ocean floor. The floatable structure includes a plurality of support or foundation members that function when in the extended position, to firmly anchor the floatable unit at a working site. This is true particularly in an unconsolidated substrate which comprises essentially a deltaic or similar composition that does not readily support an appreciable amount of weight. The foundation or caisson members are capable of being individually embedded into the ocean floor. This is achieved through the combination of a jetting system, a vacuum, or pump system, and by applying weights to the top of each caisson.
An object of the invention therefore is to provide a marine structure having deeply embeddable support or foundation members which will firmly maintain the unit at a submerged working position.
A further objective is to provide a marine structure capable of being accurately lowered to the ocean floor through the use of self contained guiding and foundation members.
A still further object is to provide a marine structure of the type contemplated including a plurality of self embedding legs or caisson-like foundation members that are conveyed on the structure and which serve to guide the unit to a working site at the ocean floor. They thereafter maintain it in place for drilling and producing operations.